Polyester high polymer synthetic paper for writing

ABSTRACT

SYNTHETIC WRITING SHEET MADE FROM A LINEAR POLYESTER AND A HIGH POLYMER HAVING A HIGHER GLASS TRANSITION POINT THAN THAT OF THE LINEAR POLYESTER BY MIXING THEM UNIFORMLY, FORMING THE POLYMER MIXTURE INTO FILM, AND STRETCHING THE FILM.

United States Patent 3,755,499 POLYESTER-HIGH POLYMER SYNTHETlC PAPER FOR WRITING Ichio Heijo, Sigehiro Wakamatsu, Hideki Wada, and Hiroshi Nagamatsu, Nagahama, Japan, assignors to Mitsubishi Jnshi Kabushiki Kaisha, Tokyo, Japan No Drawing. Filed May 24, 1971, Ser. No. 146,446 Claims priority, application Japan, May 26, 1970, 45/44,521 Int. Cl. B43] l/12; C08g 39/10 US. Cl. 260-873 Claims ABSTRACT OF THE DISCLOSURE Synthetic writing sheet made from a linear polyester and a high polymer having a higher glass transition point than that of the linear polyester by mixing them uniformly, forming the, polymer mixture into film, and stretching the film.

3,755,499 Patented Aug. 28, 1973 the flow paths for the molten resin of'the hopper, cylinder, filter, strainer, etc. of the extruder at the time of melt extrusion, it occurs that, when a transparent film is to be produced subsequently with a different kind of resin material by the same extruder, this residual inorganic material mixes with the resin material to impair the quality of the formed film. The loss in changing over of the raw material is considerable, and the extruder must be sufficiently cleaned prior to the subsequent extrusion with different kind of raw material so as to'avoid such impairment to the quality of the final product, for the purpose of which the 'extruder should unavoidably be stopped its strength, stability in sheet size, water-resistance, long preservation, and so forth. In this connection, ordinary tracing paper has fatal defects in respect of its physical strength, size stability, water-resistant property, etc. To solve such defects, use of a synthetic resin is considered most effective and advantageous, and various proposals have been made to produce a mat film which is given sufficient writability. The technique for matting the film has been developed in various ways, but there still exist various problems and difficulties with the developed techniques. One of the most diflicult problems to solve is that a sufficiently thin film is hardly obtainable, and the cost for the matting process is extremely high which hinders productivity of the mat film.

Known method for treating the film surface to render it coarse so as to give sufficient Wr-itability is grouped into two major classes of 1) sandblasting and (2) chemical treating. The disadvantageous points of the former is that the treatment speed is very slow, and the sands pierce through thin film, which renders the treatment difficult. The latter method using various alkali, and organic solvents results in the paper having coarse surface of a shallow depth which is not durable against writing with a writing article of high hardness. The paper is also lacking in size stability due to residual chemical and not suitable for preservation owing to metachromatism, etc. The method is difficult in treating a thin film which easily dissolves into the chemical.

On the other hand, there is another class of method, I

in which fine particles of inorganic material such as silica are added to a linear polyester resin as a third component, and a film obtained from this material is stretched. While the film thus obtained possesses the required mat surface, it has defects such that (1) a particular treatment is required to uniformly disperse a small quantity of the inoperation with consequent lowering of productivity.

It is therefore an object of the present inventionto provide a synthetic sheet for writing and like other purposes having improved surface condition, opacity, and other requisite properties.

It is another object of the present invention to provide an improved method of producing at high productivity and low cost, a synthetic sheet for writing, etc. having thin thickness and properly rendered non-transparent, wherein the excellent properties of the linear polyester resin is taken advantage of, and existing extruding facility and known stretching technique are fully utilized.

According to the present invention, there is provided a synthetic sheet for writing and like other purposes which consists essentially of a linear polyester and a high polymer having a higher glass transition point than that of the linear polyester at a rate of 7 to 35% by weight of the latter with respect to the total polymer mixture.

According to another aspect of the present invention, there is provided a method for producing a synthetic writing sheet which comprises mixing a linear polyester resin and a polymer having a higher glass transition point than that of the polyester resin, forming the mixture into a film, and stretching the film at a temperature above the glass transition point of the linear polyester but below the melting point thereof.

The nature, principle, and details of the present invention will become more apparent from the following tiescription and preferred examples thereof.

The linear polyester to be used in the present invention designates polyethylene terephthalate, polyethylene isophthalate, copolymer of ethylene terephthalate and ethylene isophthalate, and so forth. The glass transition temperature of amorphous polyethylene terephthalate, for

" example, is between 69 C. and 70 C. (in the case of organic fine particles having a size ranging from a few to several tens of microns over the entire surface of the film;

and (2) although the polyester film is ordinarily formed by melt-extrusion after the material is desiccated, as the inorganic fine particles added to the resin material remain, in most cases, in the drying chamber as well as in crystalline polyethylene terephthalate, it is 81 C.).

The high polymers having higher glass transition point than that of the polyethylene terephthalate are: acrylic polymers such as, for example, polymethylmethacrylate, copolymers of acrylouitrite and styrene, copolymers of acrylonitrile, butadiene, and styrene; polymers of styrene such as polystyrene; and carbonate copolymers such as 4,4-dihydroxydiphenyl-2,2 propane carbonate, etc.

The quantity of the high polymer to be added to the linear polyester depends on the kind of the polymer. A preferable range is between 7% by weight and 35 by weight of the total polymer mixture. Even when the mixing quantity of the polymer is small, the stretched film becomes semitransparent, although no proper wri-tability can be attained. On the contrary, if the mixing quantity of the high polymer is excessive, the resulting film, when it is stretched, is easily breakable and becomes diflicult to attain suficient stretching. Also, the rupture strength of the film becomes low when the stretching is carried out at a temperature lower than the glass transition point of the linear polyester. When the film is stretched at a temperature higher than the melting point of the linear polyester, the molecular chain of the polyester becomes fluidized with the result that the film does not orientate and no stretch effect can be recognized.

-..Any knownimethodfor.mixingthe polymerscan be adopted. For example, the aforementioned high polymer material in chips or powder form is mixed with the linear polyester chips at the time of film forming, and agitatedfor uniform dispersion.

Forming, into film of the mixed polymer materials may beresorted ,to by thev ordinary method, for forming the linear polyester such as melt-extrusion by T-die or annular die. r I 1 .The thus formed film is subjected to either sequential or simultaneous stretching. Stenter method, and tubular stretching method areusually employed. The stretching temperatureand the stretch ratio are also governed by the high polymer to be added to the linear polyester. Theset conditions for stretching of the linear polyester film may be followed.- The writing sheet to be thus obtained is subjected; toadjustmentfor its writability and opacity in con- .forrnityyvithits use by appropriate selection of the high polymer to be mixed, the mixing quantity thereof, the stretching temperature, the stretch ratio, method for stretching,= etc. The writability and opacity of the writing sheet according to the present invention can be first realized by the stretching, the reason for which is as follows.

As mentioned in the foregoing, the glass transition point of the amorphous polyethylene terephthalate is from 69 C. to 70 C. (81 C. in crystalline state). When a high polymer having a glass transition point higher than that of polyethylene terephthalate is mixed with the polyethylene terephthalate, formed into film, and stretched at a stretching temperature above the glass transition point of the polyethylene terephthalate and below the melting point thereof, polyethylene terephthalate assumes a rubbery state, while the high polymer added thereto is in glassy or rubbery state depending on its kind. In this case, as the glass transition point of polyethylene terephthalate is higher than that of the high polymer, polyethylene terephthalate is more preferentially stretched than the high polymer, and oriented in the vicinity of the high polymer with it as nucleus to produce irregularity on the film sur- .face, Moreover, where both polymers are dispersed uniformly, the surface irregularity of the film becomes extremely fine with the consequence that very ideal mat surface capable of producing good writing, uniform thickness of drawing lines adhered onto the sheet surface and appropriate opacity can be obtained.

The mixing high polymer may or may not have compatibility with the linear polyester, provided that it can be substantially uniformly mixed with and dispersed in the linear polyester at the time of forming, and that the formed film, regardless of whether it is transparent or not, may produce a uniform mat surface upon being stretched. It is, however, more desirable that the mixing high polymer be compatible with the linear polyester and the melting point'of both polymers be as close to each other as possible, which facilitates film forming operation.

.- The film thus obtained is heat-shrinkable, excellent in its writability, and possesses adequate opacity. In order to fur- 4. ther improve stability in the film size at a high temperature, it may be heat-treated at a temperature above the stretching temperature of the linear polyester and below the melting point of both mixing high polymer and the linear polyester.

The writing sheet according to the present invention is useful not only for writing and copying alone, but also for various purposes such as decoration paper, metal-plated paper, labels, stickers, and various other indications, as well as wrapping paper. It is particularly worthy of note that, as this writing sheet has a hardened surface by the combination of the abovementioned two sorts of polymers alone without use of any inorganic filling agent, which sur- .face is adapted to inscription with pencil of comparatively high hardness and writing ink, and given adequate opacity, and its physical properties such as rupture strength, etc. are satisfactory as aforementioned, it is best used as drawing paper. Also, as the raw material is composed of the linear polyester and the thermoplastic polymer it is uniformly melted when it is melt-extruded into film, which enables film forming operation as readily as in the case of film forming from an individual polymer material. No inconvenience in the film forming which is liable to take place due to unmelted, non-fiuidizing substance occurring in the case of adding fine particles of inorganic substance is caused. Nor, there is no problem at all with the preliminary treatments such as drying, etc. to be done in ad- Vance of the film forming operation.

Thus, it is possible that films of various sorts can be manufactured .freely by a single unit of extruder Without accompanying difiiculty in change over from one kind of polymer material to another, and the film thickness can be arbitrarily regulated, which contribute to increase in productivity, reduction in manufacturing cost, stable supply of the product to the consumer, and other remarkable effects.

In order to enable skilled persons in the art to readily practice the present invention, the following few examples are presented. It should, however, be noted that these examples are illustrative only, and that they do not intend to narrow the scope of protection afforded by the present application as recited in the appended claims.

EXAMPLE 1 Polymethyl methacrylate of varying quantities was mixed with polyethylene terephthalate, and the mixture was subjected to melt-extrusion through a T-die to form sample films of 150 microns thick of different compositional ratio. These film samples were heated to a temperature of 90 C. in a biaxial stretching machine, and then simultaneously stretched in both longitudinal (lengthwise) as wellas transverse (widthwise) directions of the film TABLE 1 Polymethyl methacrylate/polyethylene terephthalate (percent by weight) f Up to 311 hardness.

1 Up to 211" hardness.

3 Up to 411" hardness. No'rE.(1) The test for writability was carried out by using pencils manufactured by A. W. Faber-Gaston, Nuremberg, Germany and available under a trademark of Oastell." (2) The symbols 211,311, and 4H represent hardness of pencils, which are the internationally adopted symbols.

EXAMPLE 2 p A copolymer of acrylonitrile and styrene was mixed with polyethylene terephthalate at a given ratio, and the mixture was formed into film samples of 150 microns by axial stretching machine in both longitudinal and transverse directions thereof at different temperatures and stretch ratios.

The physical properties of the film samples were as a melt-extrusion through a T-die. These film sheets were 5 shown in the following Table 4.

TABLE 4 Polycarbonate polyethylene terephthalate (percent by weight) Stretch ratio (LXW) times"--. 3X3 3X3 3X3 3X3 3X3 2.5 2.5 2x2 2X2 Temperature at stretching C.) 90 90 90 90 90 90 160 Thickness of extruded film 25 25 Rupture strength (kg/cm!) 1,010 Elongation at breaking point (percent) 73 Light transmission factor (percent) 84. 3 Haze value (percent)- .u. 69. 8 Coarseness of film surface 4 Ha) 0 3 Writability- 1 Up to 213" hardness.

2 Up to H hardness.

! Up to 2H hardness.

4 Up to BE hardness.

5 Up to 4H" hardness.

H Film broken during stretching. 7 Non-uniform opacity.

stretched simultaneously by a biaxial stretching machine at a stretch ratio twice as large as the original length of the film in the longitudinal as well as transverse directions thereof at a temperature of 85 C. Also, stretch of three times as large as the original film length was also applied separately.

The physical properties of these films were as follows.

EXAMPLE 3 General purpose (GP) polystyrene was mixed with polyethylene terephthalate at different ratios, and the mixture was formed into film samples of 150 microns thick by the melt-extrusion through the T-die. These specimen films were stretched in their longitudinal direction alone by a biaxial stretching machine at 90 C. and at a stretch ratio of 3.5 times.

The physical properties of the film samples were as shown in the Table 3 below.

TABLE 3 GP polystyrene/polyethylene terephthal atc (percent by weight) S h t l n tl wise X3.5 3.5 X3.5 trete re 10 (X e g 1 23 22 22 1, 230 1, 170 910 53 32 84. 3 80.0 72.4 81. 6 83. 0 93. 1 Wrltability 1 Up o 11" hardness. 1 Up to 311" hardness.

EX A M PLE 4 Flaky polycarbonate composed of bisphenol A was mixed with polyethylene terephthalate at varying ratios given below, and the mixture was formed into film samples of microns thick by the melt-extrusion through the T-die. These specimen films were stretched by a bi- 75 1. With the polycarbonate content of 5 wt. percent, the writability and haze value are poor. The film containing 50% of polycarbonate was not rendered uniformly opaque.

As is apparent from the above table, writability as well as haze value of the film containing 10 to 35 wt. percent of polycarbonate were found as good as those of Example What we claim is:

1. A synthetic sheet for writing purposes which consists essentially of a linear polyester selected from the group consisting of polyethylene terephthalate, polyethylene isophthalate, and copolymers of ethylene terephthalate and ethylene isophthalate, and a high polymer having a higher glass transition point than that of said linear polyester at a mixing ratio of from 7 to 35% by weight of the latter with respect to the total polymer mixture, which high polymer is selected from the group consisting of a polymethylmethacrylate, a copolymer of acrylonitrile and styrene, a copolymer of acrylonitrile, butadiene, and styrene, and polystyrene, said synthetic sheet having a very finely coarsened surface due to said high polymer which is uniformly dispersed in said linear polyester to constitute the nuclei for the irregular surface thereof.

2. A synthetic sheet according to claim 1 wherein the high polymer is polymethylmethacrylate.

3. A synthetic sheet according to claim 1 wherein the high polymer is a copolymer of acrylonitrile and styrene.

4. A synthetic sheet according to claim 1 wherein the high polymer is a copolymer of acrylonitrile, butadiene and styrene.

'5. A synthetic sheet according to claim 1 wherein the high polymer is polystyrene.

References Cited UNITED STATES PATENTS 3,585,255 6/1971 Sevenich 260873 3,234,313 2/1966 Miller et al. 26423O 2,522,351 9/1950 Egolf 3566 3,640,944 2/1972 Seppala et al. 260-873 2,541,497 2/1951 Buxbaum et al. 35-66 FOREIGN PATENTS 1,539,880 8/1968 France 260-860 WELLIAM H. SHORT, Primary Examiner E. WOODBERRY, Assistant Examiner US. Cl. X.R. 

